A problem which has been encountered in the case of doors installed to be used as exterior doors, or between areas having different temperature and/or moisture environments, or where fire break-through is to be avoided, is the warping/deformation to which such doors are subjected due to its functioning as a barrier, or at least as a separating element between varying temperature and/or moisture conditions.
This problem has been investigated at length for a number of years, and has been dealt with, for example, in Swedish patent Nos. 144,418 and 339,745.
It has been established, for example, that the movement of moisture in a surface layer of wood is a factor which essentially affects the warping of a door. In an attempt to both obstruct the movement of moisture and to apply a barrier against diffusion, aluminum plates have been used on both sides of the door structure, thereby reducing both warping and the movement of moisture through the door.
If this were not done, the uncontrolled movement of moisture would lead to an increased divergence of the moisture ratio between the surface layers, and sometimes also within the door structure. In both cases, increased warping would result, and in some instances deteriorated heat insulation would also result.
The use of aluminum satisfies high demands with respect to both sheating and diffusion density. At the same time, one can also work this material with wood-working tools, and this is another decisive prerequisite in the manufacturing process.
On the other hand, one disadvantage of using aluminum is that it provides only a limited contribution to the fire-arresting qualities of the door, particularly since aluminum melts at the temperatures which can occur in such circumstances. Furthermore, the fact that one can apply commonly available tools and methods to the working of aluminum, while creating an advantage from a manufacturing point of view, does reduce the burglary protection thereof.
As the demands in terms of heat insulation increase, with the resultant use of more and more high-quality heat insulating materials, the relatively large thermal expansion of aluminum also creates problems. Improved insulation thus entails a greater difference in temperature between the surface layers, and this leads to increased warping of the door due to both temperature and moisture.
It is therefore an object of the present invention to eliminate these faults, and to introduce a door meeting high demands in terms of dimensional stability and heat insulation, as well as fire resistance and burglary protection.